The structure of organic electroluminescent devices (OLEDs) in which organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. No. 4,539,507, U.S. Pat. No. 5,151,629, EP 0676461 and WO 98/27136. In recent years, organometallic complexes which exhibit phosphorescence instead of fluorescence are increasingly being employed (M. A. Baldo et al., Appl. Phys. Lett. 1999, 75, 4-6). For quantum-mechanical reasons, an up to four-fold increase in energy and power efficiency is possible using organometallic compounds as phosphorescent emitters. In general, there are still considerable problems in OLEDs which exhibit triplet emission. For example, the physical properties of phosphorescent OLEDs are still unsatisfactory with respect to efficiency, operating voltage and lifetime for use of triplet emitters in high-quality and long-lived electroluminescent devices.
In accordance with the prior art, 4,4′-bis(N-carbazolyl)biphenyl (CBP) is frequently used as matrix material in phosphorescent OLEDs. The disadvantages are short lifetimes of the devices produced therewith and high operating voltages, which result in low power efficiencies. In addition, CBP has an inadequately high glass transition temperature. In spite of all disadvantages of CBP, it continues to be used as triplet matrix material since the problems described above have also not yet been solved satisfactorily using alternative matrix materials.
Aluminium complexes, in particular B-Alq, are furthermore known as triplet matrix materials (for example Chem. Phys. Lett. 2005, 404, 121-125).
US 2006/040139 describes the use of metal complexes with ligands based on Schiff base as host material for photoactive materials. However, only complexes with trivalent metals, in particular aluminium complexes which are penta- or hexacoordinated, are disclosed.